1. Field of the Invention
This invention relates to the production of synthesis gas and, more particularly, this invention relates to the catalytic conversion of natural gas or other forms of gaseous lower alkanes to synthesis gas by means of partial oxidation and steam reforming.
2. Description of Related Technology
As is well known in the art, synthesis gas ("syngas") is a mixture of carbon monoxide and molecular hydrogen, generally having a hydrogen to carbon monoxide molar ratio in the range of 1:5 to 5:1, and which may contain other gases such as carbon dioxide. Synthesis gas has utility as a feedstock for conversion to alcohols, olefins, or saturated hydrocarbons (paraffins) according to the well known Fischer-Tropsch process, and by other means. Synthesis gas is not a commodity; rather, it is typically generated on-site for further processing. One potential use for synthesis gas is as a feedstock for conversion to high molecular weight (e.g. C.sub.50 +) paraffins which provide an ideal feedstock for hydrocracking for conversion to high quality jet fuel and superior high cetane value diesel fuel blending components. Another potential application of synthesis gas is for large scale conversion to methanol.
In order to produce high molecular weight paraffins in preference to lower molecular weight (e.g. C.sub.8 -C.sub.12) linear paraffins, or to synthesize methanol it is desirable to utilize a synthesis gas feedstock having an H.sub.2 :CO molar ratio of about 2:1 or less. As is well known in the art, Fischer-Tropsch syngas conversion reactions using syngas having relatively high H.sub.2 :CO ratios produce hydrocarbon products with relatively large amounts of methane and relatively low carbon numbers. For example, with an H.sub.2 :CO ratio of about 3, relatively large amounts of C.sub.1 -C.sub.8 linear paraffins are typically produced. These materials are characterized by very low octane value and high Reid vapor pressure, and are highly undesirable for use as gasoline.
Lowering the H.sub.2 :CO molar ratio alters product selectivity by increasing the average number of carbon atoms per molecule of product, and decreases the amount of methane and light paraffins produced.
Thus, it is desirable for a number of reasons to generate syngas feedstocks having molar ratios of hydrogen to carbon monoxide of about 2:1 or less.
Prior methods for producing synthesis gas from natural gas (typically referred to as "natural gas reforming") can be categorized as (a) those relying on steam reforming where natural gas is reacted at high temperature with steam, (b) those relying on partial oxidation in which methane is partially oxidized with pure oxygen by catalytic or noncatalytic means, and (c) combined cycle reforming consisting of both steam reforming and partial oxidation steps.
Steam reforming involves the high temperature reaction of methane and steam over a catalyst to produce carbon monoxide and hydrogen. This process, however, results in production of syngas having a high ratio of hydrogen to carbon monoxide, usually in excess of 3:1.
Partial oxidation of methane with pure oxygen provides a product which has an H.sub.2 :CO ratio close to 2:1, but large amounts of carbon dioxide and carbon are coproduced, and pure oxygen is an expensive oxidant.
An expensive air separation step is required in combined cycle reforming systems, although such processes do result in some capital savings since the size of the steam reforming reactor is reduced in comparison to a straightforward steam reforming process.
Thus, it is desirable to lower the cost of syngas production as by, for example, reducing the cost of the oxygen plant, including eliminating the air separation step, while improving the yield as by minimizing the coproduction of carbon, carbon dioxide and water, in order to best utilize the product for a variety of downstream applications.